1. Field of the Invention
This invention relates generally to laminate magnetic thin films for data recording and more particularly to magnetic thin films having multiple ferromagnetic layers.
2. Description of the Related Art
FIG. 1 illustrates a typical head and disk system 100 including a magnetic transducer 102 supported by a suspension 104 as it flies above the disk 106. The magnetic transducer 102, usually called a “read/write head” or “slider,” may include elements that perform the task of writing magnetic transitions (the write head 108) and reading the magnetic transitions (the read head 110). The electrical signals to and from the read and write heads 110, 108 travel along conductive paths (leads), which are attached to or embedded in the suspension 104. The magnetic transducer 102 is positioned over points at varying radial distances from the center of the disk 106 to read and write circular tracks. The disk 106 comprises a substrate on which a laminate 120 having multiple layers is deposited. The laminate 120 may include ferromagnetic layers in which the write head 108 records the magnetic transitions in which information is encoded.
Extremely small regions, or bits, on the ferromagnetic layers are selectively magnetized in chosen directions in order to store data on the disk 106. The orientation of the magnetic moments of the magnetized regions is typically longitudinal. That is, the magnetic moments typically point along the plane of the laminate, rather than out of the plane. To increase the amount of data that can be stored on the disks 106 the number of bits per unit area, or storage density, must be increased.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetic moment density (Mr) (the amount of magnetic moments per unit volume of ferromagnetic materials) and the magnetic layer thickness t has decreased. Similarly, values of coercivity (Kc) and anisotropy (Ku) have also increased. However, the extent to which Mrt may be decreased and Kc and Ku may be increased is limited.
To achieve the reduction in Mrt, the thickness t of the magnetic layer has been reduced. However, as t is reduced, the magnetic layer exhibits increasing magnetic decay, attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions. One possible solution to these limitations is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
Increasing the values of Kc and Ku increases the amount of energy required to write data to the recording disk. However, write-energy requirements may not exceed the capacity of currently available write heads 108. The amount of write field required to write to a magnetic film is given by the coercive field HC which is proportional to the anisotropy field HK (approximately equal to Ku/MS for longitudinal recording media where MS is the saturation magnetization).
It is known that the write-energy requirements of a high anisotropy field and high coercivity field magnetic layers 130 may be decreased by depositing a layer 132 of thin, high magnetic moment density material with a lower HK. This high moment material is closer to the write head and more effectively couples the write field and for the proper thicknesses and materials choices can achieve “incoherent reversal.” Incoherent reversal results where the high-moment layer changes its orientation in response to an applied field and is no longer collinear with the higher anisotropy layers (130) and in turn amplifies the “torque,” or reverse field, exerted on the high-anisotropy field layer, causing it to change orientation in response to a weaker applied field than would suffice in the absence of the high-moment layer.
The high-moment layer is magnetically “soft” and can more readily change the orientation of its magnetic moment when a write-field is applied compared to the high anisotropy layer. The change in orientation of the high moment layer causes the magnetic moment of the high-anisotropy field layer to change its orientation slightly, due to the direct exchange between the two layers. It is known that for high-anisotropy field materials, the energy required to cause a change in orientation of the magnetic moments is greatest where the applied field is exactly opposite current orientation. Accordingly, the high-moment layer, by inducing the magnetic moment of the high-anisotropy field layer to shift from a direction directly opposed to the write field, reduces the amount of energy required to cause the high-anisotropy layer to reverse.
The high-moment layer also enables more effective reading and writing to the laminate by concentrating large number of magnetic moments at the top of the media. It is known that reading and writing performance increases with proximity of the transducer 102 to the media. Accordingly, the high-moment layer, due to its direct exchange coupling with the high-anisotropy layer, effectively places the signal, or stored information, in the uppermost layer of the laminate increasing the read back signal and resolution.
Prior systems attempting to achieve the benefits of a high-moment overlayer have significant drawbacks. The high-moment layers tend to have a great deal of intergranular exchange which leads to increased noise and reduced storage density. When the write head 108 applies a field causing the grains in a region 134 to transition, intergranular exchange, will cause the adjoining grains to transition. Accordingly, a larger region 136 will be affected by the write field, thereby increasing the media noise and reducing storage density. SNR is also reduced as writing one bit causes unwanted changes in adjoining bits.
In prior systems, this intergranular exchange in the high-moment layer also affects the high-anisotropy layer. The high-anisotropy layer typically also has a low Mrt and decoupled grains, which tend to reduce noise due to intergranular exchange. However, imposing a high-moment layer on the high-anisotropy field layer results in the noise of the high-moment layer being passed to the high-anisotropy layer.
In view of the foregoing, it would be an advancement in the art to provide a thin film magnetic laminate achieving incoherent reversal while avoiding a reduction in SNR or storage density. It would be an advancement in the art to provide such a film for use in commonly used longitudinal recording media.